A large part of traded goods in the world is transported over sea by container ships. The ever increasing amount of international trade results in higher demands of container terminals all over the world to process, i.e. to import, store, and/or export, more goods with an increased speed.
Container terminals, e.g. in marine ports, have the need to store containers for a time period ranging from hours to weeks. In general terms, a container yard is a container storage area in which the containers are stored in a 3 dimensional way by placing some of them on the ground beside one forming a rectangular shape and by stacking the other containers on top of those on the ground. One or more gantry cranes are used to drop-off the containers in the yard, to move the containers between locations inside the yard and to pick them up again from the yard. Different types of gantry cranes are known, in particular rubber-tired gantry (RTG) cranes and rail-mounted gantry (RMG) cranes. The invention deals with RMG cranes, where the cranes move on rails, and where the container yard is situated in between two or more of these rails so that the RMG cranes span across the yard. RMG cranes are often operating in an automated way, in which case they are also called automated stacking crane (ASC). In a container yard, each ASC may either have its own pair of rails, or the ASCs may be mounted on the same rail. In the latter case, the ASCs are not able to pass each other which in the following is denoted as non-passing cranes. In the case of two non-passing cranes, these cranes are also called twin stacking cranes or twin RMG.
In the article “Cranes with brains” by Bryfors et al, published in ABB Review March 2006, automation of a container terminal is described, where the container storage area is divided into blocks and each block is equipped with two ASCs, called automatic rail-mounted gantry cranes (ARMG). In order to control the movement of the ARMGs, each of them is equipped with a crane control system which integrates sensor information delivered from systems for target and obstacle position measurement as well as for load position measurement. Based on this information, the crane control system finds the optimal transport path within the yard, takes care of collision avoidance and keeps pendulum movements of the load as low as possible, by providing appropriate control information to underlying position and motion controllers which generate actuating signals for the drives and electric motors of the crane.
The information on which container is to be moved by which crane from which start to which end position is generated by a so-called terminal operating system (TOS). The TOS takes as input the container handling jobs required in order to unload and load the ships. Further, it considers the container handling jobs for loading and unloading the trucks or trains on the landside of the container yard. Even further, it considers the case that a target container of a job is placed beneath some other container. Then, a corresponding work-order is generated by the TOS for relocating the upper container at a nearby stack. These relocation work-orders may be called rehandling jobs.
Each ASC in the container yard receives its work orders one by one from the TOS. In case of non-passing ASCs, the work orders are executed using a first-come-first-served strategy, which means that the ASC which receives its work order first has higher priority. Accordingly, if the ASCs have colliding work orders in the sense that one ASCs work order overlaps the other ASCs position, the ASC with lower priority has to yield and wait in order to avoid collisions. These situations are in the following called interference. In order to determine which crane actually has priority and whether an interference is likely to occur or not, the crane control systems of the ASCs may exchange corresponding information.
From US 2006/0182527 A1, a method for automated container terminal scheduling is known, where it is suggested to reorganize the containers in the terminal when resources, i.e. cranes, are available to perform the corresponding tasks. The aim of the reorganization is to better adapt the locations of the containers within the terminals to external changes, such as changes in ship and/or truck arrivals, custom holds on containers or port equipment failures. Accordingly, US 2006/0182527 A1 describes a terminal operating system generating updated schedules for the various cranes and other equipment in the container terminal.
In EP 2 006 237 B1, a crane control system is disclosed for a crane that moves containers in a specific block of a container terminal. It is proposed that the crane control system takes care of the container storage optimization in its corresponding block. The tasks taken care of by the crane control system are to find an optimal placement for new containers whether delivered by land or by sea, to remove a container from the block in the shortest possible time and to relocate containers to more favorable positions during times of low crane load. Accordingly, the crane control system takes over a part of the scheduling functionality from the TOS, which in EP 2 006 237 B1 is called port logistics system.
In both documents, US 2006/0182527 A1 and EP 2 006 237 B1, the problem of non-passing ASCs is not addressed.
In the paper “Real-time scheduling of twin stacking cranes in an automated container terminal using a genetic algorithm,” by R. Choe et al, SAC '12 Proceedings of the 27th Annual ACM Symposium on Applied Computing, New York, N.Y., USA, pp. 238-243, ISBN: 978-1-4503-0857-1, the problem of scheduling twin automated ASCs is addressed. A scheduling system first identifies so called main jobs to be done at the container terminal, where the main jobs are the seaside jobs for discharging and loading the ships as well as the landside jobs for carrying in and carrying out containers from and to external trucks.
Then, the scheduling system described in Choe et al generates so called preparatory jobs, which are jobs identified to possibly, but not necessarily, assist the main jobs. The preparatory jobs are in particular rehandling jobs, as explained above, and repositioning jobs. A repositioning job is a job by which a container is moved to another position closer to its final target location. Due to the repositioning jobs, the delay caused by interference between the ASCs can be reduced, as one of the ASCs can be made to temporarily put down the container at a location outside the range of interference and to perform other jobs until the other ASC moves away.
After the generation of the preparatory jobs, the actual crane scheduling is performed in order to determine which of the jobs is to be performed in which sequence by which of the twin ASCs. For the crane scheduling, the assignment of ASCs is defined to be only required for the preparatory jobs, as in the set-up discussed in Choe et al the seaside jobs can only be performed by the seaside ASC and the landside jobs can only be performed by the landside ASC. Further, the main jobs and the preparatory jobs are treated as independent jobs. If scheduling of the preparatory jobs results in an inappropriate job order, like a repositioning job following its corresponding main job or the originally blocked container now lying on top of its stack, the respective preparatory job becomes obsolete.
Accordingly, Choe et al describes a scheduling system which expands the known functionality of a TOS by introducing repositioning jobs in order to reduce interference between non-passing ASCs. The scheduled main and preparatory jobs will afterwards be transmitted to the respectively assigned ASC and its corresponding crane control system for job execution.
In order to implement this proposed solution in an existing TOS, all the algorithms for job-handling already present would have to be replaced since the proposed scheduling represents a holistic approach. Accordingly, a considerable effort would be required for the actual testing and commissioning of the changed system. In addition, a complete change of a running system always leads to an increased risk for technical problems and system failures, at least in the beginning after the change has been affected.